HF vapor phase wafer cleaning and oxide etching

ABSTRACT

The invention provides HF vapor process conditions that can be precisely controlled with a high degree of reproducibility for a wide range of starting wafer conditions. These HF vapor processes for, e.g., etching oxide on a semiconductor substrate, cleaning a contaminant on a semiconductor substrate, removing etch residue from a metal structure on a semiconductor substrate, and cleaning a metal contact region of a semiconductor substrate. In the HF vapor process, a semiconductor substrate having oxide, a contaminant, metal etch residue, or a contact region to be processed is exposed to hydrofluoric acid vapor and water vapor in a process chamber held at temperature and pressure conditions that are controlled to form on the substrate no more than a sub-monolayer of etch reactants and products produced by the vapor as the substrate is processed by the vapor. The sub-monolayer HF vapor process regime is defined in accordance with the invention to proceed under conditions wherein no more than about 95% of a monolayer of coverage of the substrate surface occurs.

This application claims the benefit of U.S. Provisional Application No.60/118,937, filed Feb. 5, 1999, the entirety of which is incorporated byreference.

BACKGROUND OF THE INVENTION

This invention relates to processes for cleaning silicon substrates suchas silicon wafers and for etching oxide layers on such wafers, and moreparticularly relates to wafer cleaning and oxide etching techniquesemploying hydrofluoric acid.

The effectiveness of cleaning processes for removing contamination fromsilicon wafers employed for microfabrication is growing ever moreimportant as the critical size of microfabricated electronic devicesshrink. Wafer contamination is generally introduced from waferproduction and packaging, from exposure to the ambient, and from humanexposure during processing, and can consist of particles, organicresidue, adsorbed metal ions, and other contaminants. The vital role ofwafer cleaning in microfabrication processing is evidenced by the factthat about one-third of the total number of steps in a givenmicrofabrication process are cleaning steps.

To maximize microfabrication production yield, cleaning processes arerelied on to remove wafer contamination without damaging or consumingthe wafer and without introducing further contamination to the wafer.For silicon wafers, such cleaning typically includes removal of thenative silicon dioxide layer, referred to here as a native oxide layer,for brevity, that is generally present on the wafer. Metal and othercontamination can be trapped in this native oxide layer and wouldcritically contaminate high-temperature processing equipment. Removal ofthe native oxide layer is therefore generally always carried out as anintegral wafer cleaning process before any high-temperature processing awafer.

Traditionally, silicon wafers are cleaned by way of aqueous phasecleaning processes that typically employ, e.g., acids, bases, andmixtures of various chemicals. Historically, such an aqueous phaseprocess has been effective at removing contaminants and removing thenative oxide layer. Now, however, as microelectronic features shrink tothe sub-micron regime, as the aspect ratio of wafer topology greatlyincreases, and as the number of microelectronic metal interconnectlayers is increased, traditional aqueous cleaning processes are lesseffective or completely ineffective. Thorough drying of rinse solutionsfrom around and in small or high aspect ratio features can be difficultand can result in trapping of contamination at those features.Furthermore, new combinations of microelectronic materials and newexotic microelectronic materials can be adversely affected by aqueouscleaning chemicals that historically were considered benign to moreconventional materials.

Aside from structure and materials considerations, it is found thatmicrofabrication process facilities are under increasing pressure toreduce the volume of waste chemicals they generate. Wafer cleaningprocesses contribute substantially to this waste volume. Asenvironmental regulations are increased, the pressure to reduce oreliminate aqueous wafer cleaning waste will also increase.

In response to many of these issues, the use of hydrofluoric acid (HF)vapor for cleaning silicon wafers, including etching of native oxide,and etching of thicker silicon dioxide layers, has been extensivelystudied. Typically, HF vapor wafer cleaning and etching is carried outin an in situ environment and employs vapor phase HF and, e.g., vapor orliquid water, an alcohol, and other optional components such as carriergases. HF vapor etching is found to selectively etch oxide over siliconand to remove at least partially wafer contaminants.

HF vapor cleaning and etching has not been fully adopted for cleaningand etching steps in microelectronic fabrication processing, however,due to unwanted contamination that can be introduced by a vapor processitself, and due to a lack of clear understanding of the mechanisms andoperational regimes of vapor-based wafer cleaning and etching, with aresulting inability to precisely control the processes. For example, ithas been found that under some process conditions, liquid phasecondensation of vapor phase reactants on a wafer can occur during acleaning or etch process, and that high concentrations of reactionproducts in this condensed phase can result. If such reaction productsdo not desorb into the vapor phase when the process is stopped, they canproduce particulate residue on the wafer. This residue contamination ofthe wafer is often characterized by diffuse light scattering, referredto as “haze” on the wafer surface. Typically, a post-vapor clean aqueousrinse step is required to remove any such residue contamination. Thisrinsing both consumes water and produces aqueous waste volume that thevapor process was motivated to do away with, and eliminates the abilityto carry out an “all-dry” in situ process that is desirable formulti-process integration. But more fundamentally, the introduction ofadditional contaminants by a vapor process intended for contaminantremoval renders the vapor process inefficient and ineffectual.

It has also been found that under some process conditions, multilayeradsorption, rather than condensation, of vapor phase reactants can occuron a wafer during a cleaning or etch process, and that localized thickmultilayer reaction product regions can result. Such a condition isparticularly apt to occur, for multilayer process conditions, duringetching of a relatively thicker silicon dioxide layer. The reactantmultilayer regions can accelerate the local etch rate and produce alocalized piling up of reaction products, leading to pitting of theoxide layer being etched. Like the residue particulate formationdiscussed above, this oxide pitting is commonly characterized as “haze”on the etched layer surface. The lack of etch control of which thispitting is a symptom that is generally considered to disqualify thevapor etch process for microfabrication steps requiring high-precision.

Beyond the particular concerns of lack of process uniformity control andunwanted process contamination described above, it has historically beenconsidered extremely difficult to guarantee HF vapor processrepeatability or predictability with respect to starting waferconditions such as contamination conditions. These various concerns,taken together, are generally considered to outweigh the potentialbenefits that HF vapor cleaning and etching might bring tomicrofabrication process efficacy, precision, economics, andenvironmental regulatory compliance.

SUMMARY OF THE INVENTION

The invention provides HF vapor process conditions that can be preciselycontrolled with a high degree of reproducibility for a wide range ofstarting wafer conditions. These HF vapor processes of the invention canbe employed in accordance with the invention for etching oxide on asemiconductor substrate, for cleaning a contaminant on a semiconductorsubstrate, for removing etch residue from a metal structure on asemiconductor substrate, and for cleaning a metal contact region of asemiconductor substrate, among other applications.

In the HF vapor process method of the invention, a semiconductorsubstrate having oxide, a contaminant, metal etch residue, or a contactregion to be processed is exposed to hydrofluoric acid vapor and watervapor in a process chamber held at temperature and pressure conditionsthat are controlled to form on the substrate no more than asub-monolayer of etch reactants and products produced by the vapor asthe substrate is processed by the vapor.

The sub-monolayer HF vapor process regime is defined in accordance withthe invention to proceed under conditions wherein no more than about 95%monolayer coverage of the substrate surface occurs. This sub-monolayercoverage results in highly uniform, reproducible, and predictable etchand cleaning rates, such that the process is particularly robust formanufacturing scenarios, where, e.g., the oxide to be processed consistsof silicon dioxide and the substrate to be processed consists of asilicon wafer. In the sub-monolayer process regime of the invention, thepreferable etch rate of oxide is no more than about 10 Å/min.Small-sized and large-aspect ratio features are particularlywell-handled by HF vapor processes carried out in the sub-monolayerregime of the invention.

The invention provides a further process for etching oxide and cleaninga contaminant on a semiconductor substrate; here the oxide orcontaminant is exposed to a stream of frozen particles and then exposedto hydrofluoric acid vapor and water vapor in a process chamber held attemperature and pressure conditions that are controlled to form on thesubstrate no more than a multilayer of etch reactants and productsproduced by the vapor as the oxide is etched or the contaminant removedby the vapor.

The process chamber conditions can be more strictly controlled to formon the substrate no more than a monolayer of etch reactants andproducts, or further controlled to form on the substrate no more than asub-monolayer of etch reactants and products. The frozen particles canbe provided as, e.g., frozen CO₂ particles.

The invention also provides particularly well-controlled processes foretching oxide on a semiconductor substrate. In a first such process, apositive electrical charge is produced on the oxide. Then the oxide onthe substrate is exposed to hydrofluoric acid vapor and water vapor,methanol vapor, or isopropyl alcohol vapor in a process chamber held attemperature and pressure conditions that are controlled to form on thesubstrate no more than a saturated monolayer of etch reactants andproducts produced by the vapor as the oxide is etched by the vapor. Theprocess chamber conditions can be more strictly controlled to form onthe substrate no more than a sub-monolayer of etch reactants andproducts.

The positive electrical oxide charge can be produced by exposure of theoxide to an electron beam, by exposure of the oxide to ultravioletradiation through a metallic screen, and by exposure of the oxide to aplasma environment wherein the substrate is biased by anegative-polarity DC voltage.

This process can be adapted to alternatively produce a negativeelectrical charge on the oxide or to release electrical charge from theoxide. In these cases, the process chamber conditions are controlled toform on the substrate no more than a multilayer of etch reactants andproducts. The process chamber conditions can also be more strictlycontrolled to form on the substrate no more than a monolayer or asub-monolayer of etch reactants and products.

The negative electrical charge can be produced by exposure of the oxideto a plasma environment wherein the substrate is biased by a RF voltage,or by exposure of the oxide to a plasma environment wherein thesubstrate is biased by a positive-polarity DC voltage. The electricalcharge can be released by exposure of the oxide to a stream of frozenparticles, with the substrate temperature not being controlled duringthe exposure.

These various processes of the invention enable all-dry semiconductorsubstrate cleaning, oxide etching, and etch residue removal, among otherprocesses, all that conventionally require large volumes of aqueouschemicals that conventionally cannot be precisely controlled. All-dryvacuum cluster systems including chambers for the HF vapor processes ofthe invention, as provided by the invention, enable high efficiency,precision, and reproducibility for critical and frequent processesrequired of most microelectronic fabrication sequences. Otherapplications, features, and advantages of the invention will be apparentfrom the following description and associated drawings, and from theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example vapor processing systemprovided by the invention for carrying out the HF vapor processes of theinvention;

FIGS. 2A-2D are schematic views of an HF vapor process being carried outin accordance with the invention under conditions of a condensed layerregime, a multilayer adsorption regime, a saturated monolayer adsorptionregime, and a sub-monolayer adsorption regime, respectively;

FIG. 3A is a plot of measured ellipsometric angle, Y (Psi), in degrees,at a wavelength of 4052 Å, as a function of time and increasing partialpressure of HF vapor, for the HF vapor process conditions indicated,highlighting the point at which the process makes a transition from anon-condensed operating regime to a condensed layer operating regime;

FIGS. 3B-3C are plots of measured and calculated ellipsometric angle,Psi, in degrees, as a function of wavelength, for a non-condensedoperating regime and for the condensed layer operating regime,respectively;

FIGS. 4A-4B are plots of measured ellipsometric angle, Psi, in degrees,at a wavelength of 4052 Å, as a function of time, taken during the HFvapor etching of a thermal oxide layer in a non-condensed regime and inthe condensed regime, respectively;

FIG. 5 is an Arrhenius plot of measured etch rate, in Å/min, as afunction of temperature, for five different HF vapor etch conditions,which include HF vapor processes in the multilayer adsorption regime,the monolayer adsorption regime, and the sub-monolayer adsorption regimeof the invention;

FIG. 6 is an Arrhenius plot including only those points from the plot ofFIG. 5 corresponding to the monolayer and sub-monolayer adsorptionregimes, and further including calculated values for the points;

FIG. 7A is a plot of measured etch rate, in Å/min, as a function of thepartial pressure of water vapor, in Torr, for the HF vapor multilayerprocess conditions of the invention indicated;

FIG. 7B is a plot of measured etch rate, in Å/min, as a function of thepartial pressure of HF vapor, in Torr, for the HF vapor multilayerprocess conditions of the invention indicated;

FIGS. 8A and 8B are plots, produced by atomic force microscopy, ofmeasured layer topology height, in nanometers, as a function of layerextent, in microns, for an oxide layer that has been partially etched byan HF vapor process in the multilayer adsorption regime, and for anoxide layer that has been partially etched by an HF vapor process in themonolayer adsorption regime of the invention, respectively;

FIGS. 9A-9D are XPS plots of measured relative counts as a function ofbinding energy, in eV, centered on the energy corresponding to the Si2p, O 1s, 1C 1s, and F is bonds, respectively, for a native oxide layeron a silicon wafer, for a silicon wafer the native oxide layer of whichwas etched by HF vapor in the multilayer regime, for a silicon wafer thenative oxide layer of which was etched by HF vapor in the monolayerregime of the invention, and for a silicon wafer the native oxide layerof which was etched by liquid HF;

FIG. 10 is an Arrhenius plot of measured etch rate of aTetraethylorthosilicate (TEOS) layer of in Å/min, as a function oftemperature, for etch conditions in the multilayer, monolayer, andsub-monolayer HP process regimes;

FIGS. 11A-11C are plots of measured ellipsometric signal angle, Psi, indegrees, as a function of time, in minutes, for three processes in whichan oxide layer and overlaying aluminum layer are exposed to an HF vaporprocess carried out under condensed layer conditions, under multilayerconditions, and under monolayer conditions of the invention,respectively;

FIG. 12 is an XPS plot of measured relative counts as a function ofbinding energy, in eV, centered on the energy corresponding to C 1sbonds, for a thermal oxide layer, for a finger print on a thermal oxidelayer that has been subjected to a CO₂ cleaning step, for a finger printon a thermal oxide layer that has been subjected to a CO₂ cleaning stepand multilayer regime HF vapor process in accordance with the invention,and for a finger print on a thermal oxide layer that has been subjectedto a CO₂ cleaning step and a monolayer regime HF vapor process inaccordance with the invention;

FIG. 13 is an Arrhenius plot of measured etch rate, in Å/min, as afunction of temperature, for an oxide layer surface which has beenpositively precharged and for an oxide surface that has beenelectrically discharged, for two HF vapor etch conditions that includeprocesses in the multilayer adsorption regime, the monolayer adsorptionregime, and the sub-monolayer adsorption regime of the invention;

FIG. 14A is a plot of measured etch rate, in Å/min, as a function of HFvapor partial pressure, for the process conditions indicated, for anoxide layer that has been positively charged in accordance with theinvention;

FIG. 14B is a plot of measured etch rate, in Å/min, as a function of HFvapor partial pressure, including the data of the plot of FIG. 14A andfor an process not employing water vapor for etching an oxide layer thathas been positively charged in accordance with the invention;

FIG. 15 is a plot of measured etch rate, in Å/min, as a function of HFvapor partial pressure, for an oxide layer that has been electricallydischarged in accordance with the invention;

FIG. 16 is a plot of measured etch rate, in Å/min, as a function of theratio of HF vapor partial pressure to total reactant pressure, for anoxide layer that has been electrically discharged in accordance with theinvention;

FIG. 17 is a plot of measured etch rate, in Å/min for three HF vaporprocesses, one employing water vapor, one employing methanol, and oneemploying isopropyl alcohol, all for an oxide layer that has beenelectrically discharged in accordance with the invention and for anoxide layer that has been electrically positively charged in accordancewith the invention;

FIG. 18 is an Arrhenius plot of measured etch rate, in Å/min, as afunction of temperature, for an oxide layer surface which has beennegatively precharged and for an oxide surface that has beenelectrically discharged, for the HF vapor etch conditions indicated,that include processes in the multilayer adsorption regime, themonolayer adsorption regime, and the sub-monolayer adsorption regime ofthe invention;

FIG. 19 is a schematic of a vacuum cluster processing system provided bythe invention for cleaning metal contacts on a semiconductor substrateas an integral step in an all-dry multi-step process of dielectricpatterning and metal deposition; and

FIG. 20 is a schematic of a vacuum cluster processing system provided bythe invention for removing post-etch residue from patterned metal linesas an integral step in an all-dry multi-step process of metal layerpatterning and interlayer dielectric deposition.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown an example system 10 for carryingout the HF vapor processes provided by the invention. The processingsystem 10 includes a vacuum chamber 12, preferably of stainless steel. Amaterial substrate to be processed, e.g., a wafer 14, is supported on awafer platen 16, the temperature of which is preferably controlled,e.g., through a conventional water-based heating/cooling loop 18. Athrottle valve 20 is provided for maintaining the pressure of thechamber employing, e.g., a mechanical pump 22 and turbo-molecular pump24. A pressure sensor, e.g., a capacitance manometer, is provided tomeasure the chamber pressure under process conditions. Preferably, anion gauge is provided to measure the chamber pressure during evacuationwith the turbomolecular pump to assure good vacuum integrity of thechamber.

Optical ports 34, 36 of the chamber can be employed to conductellipsometric measurements 38 during a vapor process, e.g., formonitoring etch rate and film thickness in situ and in real time duringa wafer cleaning or etching process. As can be recognized, aconventional ellipsometer, e.g., a model M44 ellipsometer manufacturedby J. A. Woollam Co., Inc., of Nebraska can be employed for monitoringmultiple wavelengths. The ellipsometer can be connected to, e.g., acomputer for directing data to be analyzed by software, e.g., WVASE32,also provided by the J. A. Woollam Co., Inc.

A shower head 26 is provided for delivering process gases from a gashandling system into the chamber 12. Preferably, the showerhead isstainless steel, of a diameter of, e.g., about 2″, 4″, or other sizecorresponding to the diameter of the wafer or substrate to be processed,and includes a stainless steel screen 28 attached at its output toevenly distribute the gas flow delivery from a delivery tube 30 into thechamber. The screen can be provided with, as an example, an open area ofabout 9%, a 0.007″ hole size, and a 0.022″ hole pitch with 60°orientation. Additionally, a baffle 32 is preferably included in theshower head to block a direct stream of gas from the delivery tube 30into the process chamber 12. This configuration enables a condition ofrelatively uniform mass transport of reactants via stagnation flow,allowing relatively uniform reactant supply across the wafer andreducing product build up. Preferably, the shower head is heated toprevent condensation of reactants on the head and corrosion of the headand to better exhaust reactants.

Process gases are introduced to the delivery tube 30 through mass flowcontrollers. The delivery tube is preferably stainless steel and of,e.g., about ¼″ diameter. This tubing and shower head could also be linedwith inert materials such as Teflon® to reduce contamination of thereactants and the wafer. Water vapor is supplied directly from a watertank 42 through a mass flow controller, preferably without a carriergas. Preferably the water tank is heated to approximately 100° C. by anelectrical resistance heater such as a heating tape. Anhydrous HF isdelivered from a corresponding HF supply tank. Other gases to beoptionally employed during a vapor process, e.g., N₂, are similarlydelivered. Preferably, a heating tape 46 is provided for controlling thetemperature of the gas delivery lines and mass flow controllers so as toprevent condensation of gases in the gas delivery system. This heatingof the system also enables an increase in accuracy of gas metering andgas flow initiation and quenching, as well as inhibiting corrosion ofthe system.

In carrying out an HF vapor process provided by the invention, a waferto be cleaned or including a film to be etched is loaded into theprocess chamber and the gas delivery lines are preferably heated by theheating tape to a temperature of between about 35° C. and about 120° C.to suppress condensation of reactants on the delivery lines. Thetemperatures of the gases to be delivered are preferably maintained atbetween about 30° C. and about 100° C. by controlling the temperature ofthe lines and showerhead. The wafer platen supporting the wafer isheated to preferably maintain the wafer at a constant temperature duringthe vapor process. The chamber walls are heated to prevent condensationof the HF, water, and products on them. Sufficient heating of thechamber walls is distinguished by rapid pump down of the system afterthe process is complete. For typical processes, a chamber temperature ofapproximately 60° C. is sufficient.

The wafer temperature is preferably maintained at between about 50° C.and about 200° C., e.g., by thermal conduction to the wafer holder.Preferably, the wafer platen is designed to provide a uniformtemperature across the wafer and be in good thermal contact with thewafer; this can be achieved, e.g., by employing a anodized aluminumblock having a smooth, flat surface and through which is circulatedheating and cooling fluid. If desired, the wafer platen can be rotatedduring the vapor process to reduce the effects of mass transferresistance of process reactants and products. The gap between the showerhead and the wafer is set at between about 3 mm and about 50 mm,preferably about 12 mm, depending on the conditions of the wafer to beprocessed. An inert gas, e.g., nitrogen, is optionally flowed, at a flowrate of between about 200 sccm and about 1 liter, preferably about 500sccm, until a target pressure of between about 10 T and about 500 T,preferably about 50 T, is achieved in the chamber. This enablesstabilization of the operating chamber pressure while a pressurecontroller is on and can be particularly preferable where precisecontrol of the vapor process is desired. Otherwise, the input of thenitrogen gas need not be carried out.

Water vapor is then introduced to the chamber at a flow rate of betweenabout 5 sccm and about 100 sccm, preferably about 20 sccm. Vapor HF isthen introduced at the precise time desired to initiate a wafer etch orcleaning process, at a target flow rate of between about 10 sccm andabout 200 sccm, preferably about 50 sccm. Preferably, the HF flow rateis increased slowly from 0 to the target flow rate to prevent anovershoot of the flow rate. For the range of HF flow rates suggested,this flow rate increase can be completed typically in about 5 s to about10 s.

The flow of water vapor and HF vapor is continued for a time selected tocarry out a given cleaning or etch process, with the partial pressure ofHF at between about 5 T and about 100 T and the partial pressure of thewater at between about 2 T and about 50 T. Nitrogen flow can becontinued or quenched at the start of the vapor process as-desired.During the vapor process, ellipsometric measurements can be taken todetermine the etching rate, preferably after about 2 min to minimize anyinitial effect of the concentration of reactants near the sample at thestart of the process, and continued throughout the process as-desired.

At a selected end time for the vapor process, the gas delivery isquenched and the chamber is pumped out by the mechanical pump. Once thebase pressure of the mechanical pump is reached, the valve to that pumpis closed and the gate valve to the turbo-molecular pump is opened, forpumping to continue with this second pump. When the pressure of thechamber is below some selected level, e.g., at about 5×10⁻⁶ T or lower,the sample can be transferred from the chamber for further processing.

Control of HF vapor process parameters based on the procedures andparameter ranges given above, in accordance with the invention, enableHF vapor process operational regimes that maximize process control andrepeatability and that mininmize or eliminate particulate contaminationand pitting effects commonly associated with HF vapor processes. Theinventors herein have discovered that in general a HF vapor process canproceed under any of four different operational regimes. Anunderstanding of the distinct nature of vapor process characteristics ineach of the regimes is provided by the invention and enables precisecontrol of a selected regime.

FIGS. 2A-2D schematically depict four general HF vapor processoperational regimes identified by the inventors. The mechanisms by whichthe four regimes are formed and etch oxide on a silicon wafer dependspecifically on the partial pressures of HF and water. The first regime,depicted in FIG. 2A, is one in which vapor-phase reactants 50 and othergases, e.g., HF, H₂O, and nitrogen, condense to form a condensed,liquid-phase layer 52 on an oxide layer 54 to be etched on a siliconwafer 56. The oxide layer 54 may be a native oxide layer or a thickeroxide layer that was deposited or grown on the wafer. Such a condensedlayer forms when, e.g., for a given water vapor pressure, the selectedpartial pressure of HF, water, and product are sufficiently high thatliquid can exist in contact with the vapor. The etch process in thisregime proceeds in a manner that is substantially that of conventionalliquid-phase HF etching. Etch product residue and particulatecontamination can often occur in this etch regime. The thickness of thecondensed layer varies as a function of process time and position acrossthe wafer.

FIG. 2B depicts a second operational regime, here in which the processconditons are selected such that the vapor-phase reactants 50 form amultilayer 58 on an oxide layer 54 to be etched on a silicon wafer 56.The multilayer is produced by the adsorption of the reactant species ina manner that does not form a liquid, and is due primarily to strongbonding between a first layer and overlying layers. Typically only about2-3 layers can form before a condensed, liquid-phase layer results. Themultilayer regime possesses two sub categories: thin and thick. Thethick multilayer is formed by increased product buildup by higheretching rates or lower product removal. For given concentrations of HFand water, a thin or thick multilayer can exist, with the thickmultilayer exhibiting a higher etching rate. A thick multilayer can alsoform locally, producing areas of higher etching rate (where the thickmultilayer exists) and areas of lower etching rate (where the thinmultilayer exists). The multilayer regime formation is highly dependenton reaction species and their concentrations, transport of the productsaway from the surface, temperature, etching rates, and substratematerial.

FIG. 2C depicts a third operational regime, in which the vapor processparameters result in a condition whereby vapor-phase reactants 50 form amonolayer 60 on the surface of an oxide layer 54 to be etched on asilicon wafer 56. The term “monolayer” as used here can be defined as a“saturated monolayer,” in which the surface coverage of the oxide layerby the vapor-phase reactants approaches or is substantially at unity,but adsorption on top of this layer, i.e., formation of the multilayer,is not yet significant.

FIG. 2D depicts the final operational regime, in which the vapor processparameters result in a condition whereby vapor-phase reactants 50 form asub-monolayer 62 on the surface of an oxide layer 54 to be etched on asilicon wafer 56. The sub-monolayer regime regime can be generallydefmed as one in which no that about 95% of a monolayer exists.

The inventors herein have discovered that when an HF vapor process iscontrolled to proceed in the monolayer or sub-monolayer regimes, theoxide layer pitting, associated with processes in the multilayer regime,is reduced or eliminated. Similarly, the generation of etch productresidue and particulate contamination associated with vapor processes inthe condensed and multilayer regimes is reduced or eliminated in themonolayer and sub-monolayer regimes. Complete removal of native oxidecan be accomplished in the monolayer and sub-monolayer regimes, and bothare characterized by a substantially uniform etch rate. Wafers havinglarge-aspect ratio features can be fully cleaned and overlying oxidelayers etched without particle or residue contamination in the monolayerand submonolayer regimes because the effects of liquid surface tensionare eliminated in these regimes. Surface tension and reduced productremoval effects in recessed features can lead to locally-altered etchingrates in recessed features. Each of these characteristics will bediscussed in detail below.

EXAMPLE 1

The process parameters of a vapor-phase HF process for etching an oxidelayer of about 540 nm in thickness, grown by thermal oxidation on asilicon wafer was carried out to identify the process conditionscorresponding to the condensed regime and the uncondensed multilayer,monolayer, and sub-monolayer regimes described above. A silicon waferwas processed in the system of FIG. 1 and by the procedure describedabove. The wafer temperature was held at about 40° C., and the partialpressure of water vapor was held at about 5.4 T. The partial pressure ofHF vapor was slowly increased over time. As the partial pressure of theHF vapor was increased, real time ellipsometric measurements were takenof the reactants forming on the oxide layer surface.

Referring to FIG. 3A there is shown a plot of the measured spectroscopicellipsometric signal at a wavelength of about 4502 Å as the partialpressure of the HF vapor was increased. FIG. 3B is a plot of measuredand calculated ellipsometric signals taken over the range of wavelengthsshown, for the conditions to the left in FIG. 3A, where the HF vaporpartial pressure was less than about 33 T. FIG. 3C is a plot of measuredand calculated ellipsometric signals taken over the range of wavelengthsshown, for the conditions to the right in FIG. 3A, where the HF vaporpartial pressure was greater than about 33 T.

It was found that for the temperature and water vapor conditionsselected, at an HF vapor partial pressure of about 33 T, a condensedlayer of reactants started to form on the oxide layer. Before theoccurrence of this condition, the ellipsometric measurements of FIG. 3Agradually increased and then decreased as the oxide film was etched asthe HF vapor partial pressure was increased. Once a condensed layerbegan to form, the ellipsometric measurements were found to increase infrequency, indicating rapid etching as the HF vapor partial pressure wasincreased beyond the condensation point.

As shown in FIG. 3B, the calculated ellipsometric signal values andmeasured values across a range of wavelengths were found to be wellfitted by a model for etching of an oxide layer over silicon wherein nocondensed layer was formed. Conversely, as shown in FIG. 3C, thecalculated ellipsometric signal values and measured values were found tonot fit a model for etching of a simple oxide layer over silicon,because of the formation of a condensed layer over the oxide withvarying thickness. It is understood that for the non-condensed regimes,vapor-phase molecules adsorbed on the oxide surface did not impact thecorrectness of the ellipsometric measurement, leading to its goodagreement with calculated values. For the condensed regime, thenonuniformity of the condensed layer, as it was formed and grew, set upa condition of interference with the ellipsometric measurement,resulting in unpredictable signal values. Therefore, the ellipsometricmeasurements were shown to be a sensitive metric of whether a condensedfilm was present.

EXAMPLES 2-3

A layer of oxide of about 5500 Å in thickness, grown by thermaloxidation on a silicon wafer was etched following the HF vapor etchprocedures given above, with the conditions set such that the etch wascarried out in a noncondensed regime. FIG. 4A is a plot of ellipsometricspectroscopic signal for a wavelength of 4502 Å taken during the etchprocess. As the thickness of the oxide layer decreased due to the etch,the signal increased in an expected manner. The etching process wasstopped at 3 min after etching about 300 Å of the oxide.

A layer of thermal oxide of about 5500 Å in thickness was also etchedfollowing the HF vapor etch procedures given above, here with theconditions set such that the etch was carried out in the condensedregime. FIG. 4B is a plot of ellipsometric spectroscopic signal for awavelength of 4502 Å taken during the etch process. As the thickness ofthe oxide decreased and the condensed layer is formed, the measuredsignal displayed the characteristic interference due to the etching ofthe oxide layer; however, the varying thickness of the condensed layercomplicates the thickness measurement. The complete etch of the layerwas completed in about 0.65 min, as indicated in the plot.

Examples 1-3 demonstrate that one can identify the state of an HF vaporprocess as being in the condensed or one of the noncondensed regimesbased on the degree of correspondence between calculated and measuredellipsometric signal values and the behavior of the signal as an etchproceeds. Condensed layer etching can be characterized by anellipsometric signal that displays high rates of etching via theinterference cycles and the time for completion and the inability to fitto a thin film model for multiple ellipsometric wavelengths because ofnonuniformity in a condensed layer. The multilayer regime cannot beeasily distinguished from the ellipsometric measurements, but can bedistinguished by its characteristic reduction in etching rate withincreasing temperature between the condensed and the monolayer regime.

EXAMPLES 4-8

The impact of temperature and pressure on oxide etch rate was measuredfor HF vapor processes carried out in the sub-monolayer, monolayer, andmultilayer noncondensed regimes to further characterize the distinctionsbetween the three regions. Referring to the Arrhenius plot of FIG. 5,vapor HF oxide etches were carried for a range of temperatures and forfive different partial pressure conditions. Each of the wafers wassubjected to a pre-etch step in which a positive charge was produced onthe oxide layer. Details of this charging step and its impact on theetch characteristics are provided below.

In all cases, a layer of thermal oxide of a thickness of about 5500 Åwas exposed to the etch conditions for about 6 min. The total gas flowrate for all of the etch processes was set at 500 sccm. For the caseswhere the HF vapor partial pressure was 20 T, 15 T, or 10 T and thewater vapor partial pressure was 8 T, 6 T or 4 T, respectively, thetotal pressure was 125 T. For the case where the HF vapor partialpressure was 5 T and the water vapor partial pressure was 2 T, the totalpressure was 62.5 T. For the case where the HF partial pressure was 2.5T and the water vapor partial pressure was 1 T, the total pressure was31.8 T. Ellipsometric measurements were taken during each etch processto determine the corresponding etch rate.

The plotted etch rate trends enable a clear recognition of the threenoncondensed layer regimes. In the sub-monolayer regime, for a giventemperature, as the reactant partial pressures are increased, the etchrate increases but the effect of increasing pressure diminishes as themonolayer conditions are approached. The etch rate thus asymptoticallyapproaches a saturation point in the etching rate as the pressure isincreased. This is consistent with an understanding that the etch ratein the sub-monolayer regime is impacted by the amount of adsorbedreactant molecules. Generally, for given partial pressures, as thetemperature is increased, the sub-monolayer etch rate also increases.

The saturated monolayer regime is distinguished by its positive apparentactivation energy, with a positive Arrhenius response, reflected by thedownward slope of the dotted line on the Arrhenius plot in FIG. 5 andthe corresponding increasing etch rate with increasing temperature. Thistemperature dependence is stronger than that in the sub-monolayerregime, which does not follow a true Arrhenius response in temperaturebut demonstrates a positive response with temperature increase. The etchrate of the saturated monolayer regime is relatively independent of thepartial pressures when their ratio is maintained constant; this isconsistent with the distinction that the monolayer regime defines acondition wherein the oxide surface is substantially covered and changesin the partial pressure do not greatly affect the coverage.

The multilayer regime is distinguished from the monolayer regime by anegative Arrhenius response to temperature increase; as the temperatureis increased, the etch rate decreases. In the multilayer regime for agiven temperature, as the pressure is increased, the etch rateincreases. From these observations, it is found that the monolayerregime can be identified by a weak dependence on pressure, with themultilayer regime distinguished by a strong dependence on pressure. Themonolayer regime further is distinguished by a first increase in etchrate with increasing temperature at a temperature greater than that atwhich a negative etch rate-temperature dependence is demonstrated, whichcharacterizes the multilayer regime. The sub-monolayer regime is thencharacterized by temperatures that are higher than the first temperatureincrease that indicates a shift from the multilayer to the monolayerregime. Furthermore, a positive Arrhenius temperature response isdemonstrated in the sub-monolayer regime approaching the monolayerregime, but a negative Arrhenius response is demonstrated in themultilayer regime approaching the monolayer regime. Finally, the etchrates of the sub-monolayer and monolayer regimes are lower than the etchrates of the multilayer regime at the same temperatures.

The proportionality between HF and water vapor partial pressure and etchrate in the monolayer and sub-monolayer regimes for a reasonable rangeof process conditions enables the modeling of the oxide etch rate in themonolayer and sub-monolayer regimes based on Langmuir-Hinshelwoodkinetics, where the etch rate is assumed to be roughly proportional tothe fraction of HF and water vapor that is adsorbed on an oxide surface.Using this model, and based on the experimental data given above, theoxide etch rate, E. R., in the sub-monolayer and monolayer regimes canbe given as: $\begin{matrix}{{E.R.} = \frac{\begin{matrix}{1.059 \times 10^{12}^{{- 15500}/{RT}} \times 1.79 \times 10^{- 9}^{13000/{RT}}P_{HF} \times} \\{9.107 \times 10^{- 8}^{10500/{RT}}P_{H_{2}O}}\end{matrix}}{( {1 + {1.79 \times 10^{- 9}^{13000/{RT}}P_{HF}} + {9.107 \times 10^{- 8}^{10500/{RT}}P_{H_{2}O}}} )^{2}}} & (1)\end{matrix}$

where E.R. and the pre-exponential factor are expressed in Å/min; whereP_(H20) is the partial pressure of the water vapor and P_(HF) is thepartial pressure of the HF vapor, both in Torr; R is the ideal gasconstant, T is temperature, and where all activation energies areexpressed as Kcal/mol.

The Arrhenius plot of FIG. 6 replots the data from FIG. 5 for themonolayer and sub-monolayer regimes, along with the-etch rate valuescalculated for each condition based on expression (1) given above. Thegood fit between the calculated and measured values validates theassumption discussed above with regard to the dependence of thesub-monolayer regime on fraction of adsorbed reactants. The oxide etchrate of the sub-monolayer and monolayer regimes is found from the plotof FIG. 6 to be between about 10 Å/min and about 100 Å/min.

Because the etch rate is approximately proportional to the coverage ofthe surface in the sub-monolayer and monolayer regimes for a fixed ratioof partial pressures of the HF and water, the etch rate can be used todetermine the approximate coverage of the surface at a fixedtemperature. For example, given an etch rate of 100 Å for the monolayercoverage, a measured etch rate of 95% of the value would reflect a 95%coverage of the surface by the reactant. The invention defines thesub-monolayer regime as including up to about 95% surface coverage. Thisetch rate-coverage correspondence is thus found to effectively enabledistinctions between the sub-monolayer and monolayer regimes.

EXAMPLES 9-10

The oxide etch characteristics of the multilayer regime were furtherinvestigated to provide identifying distinctions of this regime. Thereis shown in FIG. 7A a plot of oxide etch rate as a function of thepartial pressure of water vapor, for a water temperature of 40° C., atotal flow rate of 500 sccm, a total pressure of 205 T, and an HF vaporpressure of 7 T. These measurements were made on thermal oxide samplesof about 5500 Å in thickness that were etched for several minutes whilethe thickness were measured using ellipsometry. The change in thicknessper minute (etch rates) were computed from this data. From the plot itis seen that the multilayer regime is characterized by an oxide etchrate that is substantially linearly proportional to water vapor partialpressure.

Similarly, referring to the plot of FIG. 7B, it is found that for apartial pressure of water vapor of 4 T, a temperature of 40° C., a totalflow rate of 500 sccm, and a pressure of 250 T, the multilayer regime ischaracterized by a generally proportional response to increases in thepartial pressure of HF vapor. It is noted, however, that above an HFvapor partial pressure of about 35 T this linearity fails. Such isunderstood to be caused by a transition from the thin multilayer regimeto a thick multilayer regime. At higher water partial pressures thanshown in FIG. 7A, a similar transition is expected. At even higher waterand/or HF partial pressure than shown in FIG. 7B, the condensed layerregime would exist. The multilayer regime including both the thin andthick regions at a given temperature lies between monolayer andcondensed regimes having higher partial pressures of HF and/or thebordering monolayer regime, but lower partial pressures than thebordering condensed regime.

Having clearly distinguished the characteristics of the condensed,multilayer, monolayer, and sub-monolayer HF vapor process regimes, theinventors herein have discovered that many of unwanted effects commonlyassociated with HF vapor processing can be eliminated if the process iscarried out in either the monolayer or sub-monolayer regime. For manyapplications, the sub-monolayer regime is preferred.

Considering first the common occurrence of oxide layer pitting and hazeduring HF vapor etching, as discussed earlier, such pitting is typicallyassociated with conditions in the multilayer regime and is the result oflocalized formation of thicker multilayer regions that accelerates thelocal etch rate.

EXAMPLE 11

A layer of oxide of 5500 Å in thickness, and thermally grown, was formedon each of two silicon wafers. Both of the oxide layers were subjectedto an electrical charging step, described in more detail below. One ofthe oxide layers was etched in an HF vapor etch process under multilayerconditions of 40° C., 125 T total pressure, 500 sccm total gas flowrate, 10 T HF vapor partial pressure, and 4 T water vapor partialpressure. The other oxide layer was etched in an HF vapor etch processunder monolayer conditions of 90° C., 125 T total pressure, 500 sccmtotal gas flow rate, 10 T HF vapor partial pressure, and 4 T water vaporpartial pressure. As shown in FIG. 6, the etching rate predicted by theLangmuir-Hinshelwood model for this condition lies near the border ofthe monolayer and sub-monolayer regimes. For both the monolayer andsub-monolayer regimes, the samples were observed to be specular, i.e.not exhibiting hazing which is caused by pitting or particulatecontamination of the surfaces.

Each of the etch processes was suspended after 200 Å of oxide had beenetched, as determined by ellipsometric measurements. Atomic forcemicroscopy (AFM) was then carried out to analyze the surface morphologyof the partially etched oxide layers.

FIG. 8A is the AFM profile produced for the sample etched in themultilayer regime and which exhibited hazing, and FIG. 8b is the AFMprofile produced for the sample etched under the conditions indicatedabove and which was specular in appearance. Note that themultilayer-etched sample exhibits substantial pitting, producing surfacehaze. The elevated feature to the sides of the pitting is an AFMartifact. The size of each pit is about 10,000 Å in diameter and betweenabout 100 Å-200 Å in depth, confirming that each pit is not a deeptrench but a recessed area that would be produced by a locally high etchrate.

The profile of the monolayer-etched sample does not exhibit any pitting;uniform etching across the entire wafer is observed. Similar results canbe achieved for conditions in the sub-monolayer regime—both themonolayer and sub-monolayer regimes are found to not produce surfacehaze. A final aqueous rinse step is conventionally required to removethe buildup of products on the surface which can occur for samplesetched in the condensed regime and under some circumstances in themultilayer regime. The invention eliminates the need for such byemploying monolayer or submonolayer etch conditions that suppress thepitting mechanisms. Similarly, the invention reduces the need for waferrotation during an etch process to enhance the mass transfer rate or toenhance the uniformity of mass transfer across an etching layer; evenwithout such enhancements, the monolayer and submonolayer etch regimesof the invention are found to provide more uniform etch results.

EXAMPLE 12

Native oxide removal is a critical requirement for complete cleaning ofsilicon wafers, as explained above. The effectiveness of multilayer andmonolayer HF vapor etching of native oxide silicon layers was comparedwith that of aqueous HF etching. Three silicon wafers, each with anative oxide layer having a thickness of about 20 Å were subjected toexposure to either a liquid-phase HF etch, a multilayer HF vapor etch,or a monolayer HF vapor etch. The liquid-phase HF was carried out with a10:1 diluted HF solution. For this aqueous process, the wafer wasimmersed in the diluted solution for 1 min, followed by a dionized waterrinse and conventional spin drying. Hydrophobicity was observed visuallyon the silicon surface at the completion of the liquid-phase etch.

The multilayer native oxide etch was carried out under conditions of 40°C., 125 T total pressure, 500 sccm total gas flow rate, 10 T HF vaporpartial pressure, and 4 T water vapor partial pressure. The monolayernative oxide etch was carried out under conditions of 90° C., 125 Ttotal pressure, 500 sccm total gas flow rate, 10 T HF vapor partialpressure, and 4 T water vapor partial pressure. These conditions borderthe monolayer and sub-monolayer regimes. Because the kinetics of themonolayer and sub-monolayer regimes are similar, similar results arefound to be obtained for both regimes, but with lower etching rates andbetter control in the sub-monolayer regime.

Referring to FIGS. 9A-9D, there are shown the relevant XPS spectrameasured at the completion of the native oxide etches. The multilayerand monolayer HF vapor processes were found for all considerations to besuperior over a liquid-phase process. The Si—O peak was still observableafter liquid HF etching, but not found for either of the HF vaporprocesses. Unlike the vapor-etched wafers, the wafer etched in theliquid-phase HF exhibited substantial oxygen after the etch. This resultcould have been due to imperfect removal of the native oxide by the etchor due to immediate native oxide regrowth by exposure to the ambient.

The absolute amount of carbon was reduced after all of the etchprocesses; but carbon contamination was observed for the aqueousprocess, at a higher binding state, indicating that a fluorocarboncontaminant was likely contained in the HF etch solution. Finally, thefluorine remaining on the wafer after the native oxide etch was found tobe dramatically higher for the liquid-phase etch than for thevapor-phase etches. These comparisons illustrate that contamination canbe minimized during native oxide removal through the use of multilayer,preferably monolayer, and most preferably sub-monolayer, rather thanliquid-phase, HF processes.

EXAMPLES 13-15

Tetraethylorthosilicate (TEOS) is commonly employed as an interlayerdielectric material between metal lines due to the ability to depositTEOS layers at relatively low temperatures, thereby adding little to thethermal budget of a semiconductor fabrication sequence. Cleaning ofexposed metal and semiconductor contact areas is required to removeoxides on the metal and substrate and enable the formation of a via toan adjacent metal line. Such cleaning is required to be done with theinterlayer metal dielectric exposed.

The etch characteristics of TEOS were investigated for multilayer,monolayer, and sub-monolayer HF vapor process regimes. Layers of TEOSwere deposited by plasma enhanced chemical vapor deposition on siliconwafers. No annealing of the films was carried out.

HF vapor etching of the TEOS films was carried out for three differentpartial pressure combinations of HF and water vapor; 5 T and 10 T forthe HF vapor, and 4 T and 8 T for the water vapor, as indicated on theplot. The total pressure of about 125 T was used for these processeswith total flow rates of about 500 sccm.

FIG. 10 is an Arrhenius plot of the etch rate of the TEOS layers as afunction of temperature for the three partial pressure combinations.Ellipsometric measurements were employed to determine the plotted etchrates. Comparing this plot with the Arrhenius plot of FIG. 5, it isfound that the TEOS and thermal oxide film vapor etch mechanisms followsubstantially identical trends. In the monolayer regime, the TEOS isfound to etch about 2-3 times faster than thermal oxide for the sameprocess conditions, while in the multilayer regime, the TEOS is found toetch about 10 times faster than thermal oxide for the same processconditions.

Low etch selectivity between TEOS and thermal oxide is generally favoredfor cleaning metal contacts, while high selectivity is often desired forselectively removing TEOS from a site. The monolayer and submonolayerregimes are thus found to be well-suited for metal contact cleaning. Itis expected that if the TEOS films employed in these experiments hadbeen densified by a heat treatment such as annealing, an even lowerselectivity between TEOS and thermal oxide would have been obtained.

EXAMPLES 16-18

Any process that is performed for cleaning metal contacts inherentlyexposes the metal material, which commonly is aluminum. This isspecifically the case for the removal of polymer residue that can remainat the edges of a metal line after a plasma etching process is carriedout to pattern metal lines with a photoresist mask. Frequently, polymerresidue remains at the edges of an etched metal line even after a plasmaashing procedure is completed. The polymer residue must be removed priorto subsequent processing as it would critically contaminate processingequipment. Liquid-phase diluted HF solution is known to be a goodpolymer remover, but it corrodes and etches aluminum rapidly.

The degree of attack of a thin aluminum layer by vapor HF etch processeswas investigated for the condensed layer, multilayer, and monolayerregimes. For each regime, a layer of thermal oxide of about 5500 Å inthickness was formed on a silicon wafer and a layer of aluminum having athickness slightly less than about 20 Å was deposited on the oxidelayer. This thickness was selected because of its transparency forenabling determination of the thickness of the underlying oxide layer byellipsometric measurement. The aluminum deposition was performed in thesame vacuum cluster processing system with vacuum transfer betweenchambers, thereby preventing oxidation of the aluminum by air.

For the condensed layer regime, the process conditions of the etch wereset at a temperature of 40° C., a total pressure of 125 T, a total flowrate of 500 sccm, an HF vapor partial pressure of 40 T, and a watervapor partial pressure of 10 T. For the multilayer regime, the processconditions of the etch were set at a temperature of 40° C., a totalpressure of 125 T, a total flow rate of 500 sccm, an HF vapor partialpressure of 10 and a water vapor partial pressure of 4 T. For themonolayer regime, the process conditions of the etch were set at atemperature of 90° C., a total pressure of 125 T, a total flow rate of500 sccm, an HF vapor partial pressure of 10 T and a water vapor partialpressure of 4 T.

FIG. 11A is a plot of the ellipsometric signal at a wavelength of 4052 Åas a function of time for the condensed layer etch process. From theplot it was determined that the oxide underlying the aluminum layer wasetched immediately upon exposure to the condensed regime conditions, andwas etched at a rate of about 10,000 Å/min. This result corresponds tothat of aqueous HF and indicates the incompatibility of condensed layerprocessing with exposed aluminum.

FIG. 11B is a plot of the ellipsometric signal at a wavelength of 4052 Åas a function of time for the multilayer etch process. An initial etchinhibition period of just about 3 min was followed by oxide etch at arelatively slow rate. This suggests that for the multilayer regime, thethin aluminum layer was not etched as rapidly as in the condensed layerregime, but that that HF molecules adsorbed on the aluminum layerpenetrated the layer, resulting in a slow etch of the oxide.

FIG. 11C is a plot of the ellipsometric signal at a wavelength of 4052 Åas a function of time for the monolayer etch process. As indicated inthe plot, the signal changed less than about 0.2 degree for the entire 6minute-long etch process. This suggests that neither the aluminum layernor the underlying oxide layer were damaged by the monolayer HFexposure, and therefore that a monolayer HF vapor process can safely beemployed to remove residue after metal patterning. The elimination of arinse step through the use of a vapor process is also benefit inreducing the potential of metal corrosion. It is also understood thatthe HF vapor process could provide a further benefit by inherentlyproducing a protective fluorinated layer on the surface of the metal.

Turning to further aspects of the invention, there is provided anoptional first step of cleaning a wafer or an oxide layer to beprocessed by an HF vapor process with an ice jet, e.g., a jet of CO₂:CO₂ ice jet wafer cleaning is a known surface cleaning method employinga high-velocity stream of carbon dioxide directed at a wafer surface.This procedure can be employed for at least partially removing carboncontamination as well as particulates from a wafer surface.

The inventors herein have determined that carbon and particulatecontamination does not substantially impact the etch rate and etchuniformity of HF vapor processes carried out in the condensed layer andmultilayer processing regimes, as the etching kinetics in these regimesare less sensitive to surface contamination. In contrast, etchmechanisms carried out in the sub-monolayer and monolayer regimes can besubstantially inhibited or enhanced by carbon and particulatecontamination, both in an unpredictable manner. This is due to the highdegree of sensitivity of the sub-monolayer and monolayer regimes to thechemical and electrical characteristics of species adsorbed on thesurface, given that these regimes are surface reaction limited. As aresult, the inventors have recognized that the repeatability ofmonolayer or sub-monolayer HF vapor processes carried out oncontaminated wafers or oxide layers can be very poor.

The inventors have discovered the surprising result that an initial icejet cleaning of a carbon-contaminated wafer or oxide layer, even if notcomplete, reduces the contamination to a level that does not impact theefficacy or uniformity of etch processes in the sub-monolayer andmonolayer regimes. In other words, it has been discovered that contraryto conventional wisdom, the surface-reaction limited sub-monolayer andmonolayer regimes can proceed on a contaminated surface in a uniform andpredictable manner, if the surface is first exposed to an ice jetcleaning procedure. The invention thus provides for ice jet cleaningprocesses using CO₂, Ar, or other suitable gas to prepare a wafer oroxide layer for cleaning or etching in the monolayer and sub-monolayerregimes.

The ice jet cleaning step of the invention consists of directing ahigh-velocity stream of small dry ice crystals toward a wafer or oxidesurface. Such crystals can be formed by, e.g., the expansion ofhigh-pressure liquid or gaseous carbon dioxide from a small orifice.Adiabatic expansion of the liquid or gas generates a temperature dropwithin the orifice that is sufficient to produce nucleation of small dryice particles. Preferably, there is provided for this expansion a sourceof carbon dioxide liquid or gas, e.g., gas at a pressure at about 830psi. It can be preferable to provide a dip tube that allows withdrawalof the CO₂ directly from inside a gas storage cylinder, but otherarrangements are also suitable.

A wafer to be cleaned by the ice particles is positioned on, e.g., analuminum sample holder on which the wafer is held by, e.g., a vacuumsuction contact. For some wafer cleaning processes, it can be preferableto prevent condensation of water on the wafer due to excessive coolingby the Co₂; such can be accomplished by, e.g., providing a wafer holderthat is thermally conducting and that is connected to, e.g., a heatingblock for heating the wafer during the cleaning step.

In carrying out the ice jet cleaning procedure, it is preferable tofirst initiate the ice jet formation out of the nozzle with the nozzledirected away from the wafer to be cleaned. Once an ice stream is formedand stabilized, the ice jet stream is then preferably swept from side toside across the entire wafer area. An incident ice jet stream angle ofbetween about 30° and about 45° to the normal to the surface, and anozzle-to-surface distance of between about 1″and about 1.5″, ispreferred. For a 4″ wafer, ice jet exposure of the entire wafer shouldtake between about 3 min and about 4 min. It is recognized that the useof a gas knife nozzle, in which a broad source of the ice jet can beproduced, can be employed to enable a more rapid processing of thewafer.

The force of adhesion between submicron-sized particles and a wafer oroxide surface is typically much larger than the drag force ofconventional fluid-based cleaning. But for the ice jet cleaning process,the cleaning mechanism is primarily by momentum transfer between theejected ice particles and contaminants on the wafer surface. As aresult, the small ice particles from the jet stream can overcome theparticle adhesion force and free the particles from the surface by thismomentum transfer.

It is also found that the ice jet stream sweeps away removed particlecontamination, removing it from the wafer environment. Thick deposits ofhydrocarbon are partially removed by interaction of the ice particleswith the ice jet crystals, but some hydrocarbon contamination generallyremains after ice jet processing. Similar mechanisms for particulatecontamination removal and hydrocarbon removal take place in ice jetscreated from other gases such as Argon, therefore, their use as apretreatment for the monolayer and sub-monolayer regimes is contemplatedby the invention. Additional details for carrying out ice jet processingare provided by Tamai et al., in U.S. Pat. No. 5,512,106, Apr. 30, 1996;by Whitlock et al., in U.S. Pat. No. 4,806,171, Feb. 21, 1989; and byOhmori et al., in U.S. Pat. No. 5,147,466, Sep. 15, 1992; the entiretyof all of which are hereby incorporated by reference.

EXAMPLE 19

A layer of thermal oxide of about 5000 Å in thickness was formed on eachof two silicon wafers and a finger print was made on the oxide surfaceof each. Prior to making the finger print, the carbon 1s XPS spectra ofthe two oxide surfaces was profiled. With the finger print in place, thetwo oxide layers were subjected to a CO₂ ice jet cleaning step followingthe procedures given above. The XPS spectra was then profiled. One ofthe wafers was then subjected to a multilayer HF vapor process, atconditions of HF vapor partial pressure at 10 T, water vapor partialpressure at 4 T, total pressure at 125 T, flow rate at 500 sccm, andtemperature at 40° C. The other wafer was subjected to a monolayer HFvapor process, at conditions of HF vapor partial pressure at 10 T, watervapor partial pressure at 4 T, total pressure at 125 T, flow rate at 500sccm, and temperature at 90° C. Each of the HF vapor processes werecarried out for 2 minutes, etching about 200 Å of oxide during thattime.

Each of the oxide layers was inspected at the completion of the CO₂cleaning step and at the completion of the HF vapor processing steps. Itwas found that remnants of the finger print remained using XPS on theoxide layer after the CO₂ cleaning step. It was surprisingly found thatthe remnant of the fingerprint did not alter the removal of theunderlying oxide layer processed in the monolayer HF vapor regime andthat the oxide layer was uniformly etched in the monolayer regime afterthe ice jet exposure. In fact, such contamination was found to beremoved by the HF process in both the monolayer and sub-monolayerregimes. This validates the unexpected discovery by the inventors thatthe ice jet cleaning process synergistically cooperates with thesurface-reaction limited monolayer and sub-monolayer HF vapor processesto enable uniform etching of those processes even in the presence ofsurface contamination.

In general, monolayer and sub-monolayer surface kinetics aresignificantly affected by contaminants such as hydrocarbons that blockthe surface preventing adsorption of the reactants. Thick carbonaceousdeposits have been reported to accelerate the etching rate of underlyingoxides by absorption of the reactants within the carbonaceous deposit.It is therefore surprising that the contamination left after CO₂cleaning did not alter the etching of the underlying oxide. Thisdiscovery indicates that the general benefits of processing in thesub-monolayer and monolayer regimes can be extended even to wafers andoxide layers having carbon contamination, in contradiction totheoretical prediction.

FIG. 12 is an XPS plot of measured relative counts for the energy regioncorresponding to carbon Is from the finger print. The carbon peak isshifted to 290 eV from its expected energy of 285 eV due to anelectrical charging effect of the XPS measurement. The plot highlightsthe finding that the monolayer regime was found to be more effectivethan the multilayer regime at cleaning residual carbon remaining afterthe ice jet step. In combination with the etch uniformity visuallyconfirmed, it is understood in accordance with the invention that thesurface-reaction limited monolayer and sub-monolayer process regimes aresuperior for providing etch control and uniformity even in the presenceof carbon contamination. This finding indicates that the sub-monolayerand monolayer HF vapor processes of the invention are particularlywell-suited for removing polymeric residue from a plasma-etched layer,given that the main constituents of such residue are carbon and silicondioxide.

The invention provides further techniques for controlling HF vaporprocess mechanisms. Specifically, there is provided the parameters foran initial step of electrically charging or discharging an oxide layersurface to control the etch rate of the oxide layer by an HF vaporprocess in a repeatable, predictable manner. Handling history, cleaningprocesses, ambient conditions, and many other factors are recognized toall impact the electrical charge state and corresponding HF vapor etchrate of an oxide layer. The prespecified and controlled chargepreparation step provided by the invention compensates for such factorsto render an HF vapor oxide etch process predictable and repeatable.

The impact of a particular oxide layer charge polarity on etch rate ofthe oxide layer is dependent upon the particular operating regime inwhich the HF vapor process is set to proceed. Two etch states aredefined in accordance with the invention: a nonactivated, low-etch ratestate, produced by an initial electrical discharge step, and anactivated, high-etch rate state, produced by an initial electricalcharging step. Given the demonstrated benefits of the non-condensedsub-monolayer, monolayer, and multilayer HF vapor regimes and theirpreference in accordance with the invention, the invention providesparticular charging and discharging parameters for these particularregimes.

The inventors herein have discovered that for any of the threenon-condensed regimes, a nonactivated, low-etch rate state of a givenregime can be predictably carried out on an oxide layer by firstdischarging any static charge build-up that might be present on theoxide layer surface. For the sub-monolayer and monolayer regimes, anactivated, high-etch rate state process can be carried out on an oxidelayer by first electrically charging the oxide surface with a positivepolarity charge. Intermediate charging of the surface can produceetching rates that have intermediate etching rates. For the multilayerregime, an activated, high-etch rate state process can be carried out onan oxide layer by first electrically charging the oxide surface with anegative polarity charge. Each of these techniques are described in turnbelow.

Considering first the sub-monolayer and monolayer regimes, a positiveelectrical charge can be imparted to the surface of an oxide layer to beetched in one of these two regimes by a range of techniques provided bythe invention. In a first technique, the oxide layer surface is exposedto a conventional electron beam irradiation scan. For example, ahigh-energy electron beam, characterized by, e.g., a 10 KeV acceleratingvoltage and 25 μA emission current, with a spot size of about 1 cm indiameter, can be employed to irradiate an oxide surface. The beam isscanned across the surface, for a total irradiation time of, e.g., about30 minutes. The electron beam scan is understood to charge the oxidesurface with a positive polarity due to generation of secondaryelectrons from the oxide surface.

Exposure of an oxide layer surface to UV irradiation can also beemployed for producing a positive electrical charge on the surface. Inaccordance with the invention, a UV source is directed toward an oxidesurface through a metal screen that is electrically biased by a DCvoltage source. The screen is preferably separated from the oxidesurface by a gap of about 1 inch. The polarity of the DC bias has noeffect on the electrical charging process. It is understood that the UVsource introduces sufficient photon energy to excite and free thesurface electrons of an oxide layer, resulting in a positively chargedsurface state.

A positive surface charge can also be imparted to an oxide layer byprocessing the layer in a plasma chamber. In a first plasma chargingprocess provided by the invention, a wafer including an oxide layer issupported on the lower metal electrode of the chamber with the waferinsulated from the electrode by an insulating layer such as Kapton tape,a configuration used to electrostatically clamp the wafer to theelectrode. The DC power supplied to the lower electrode is set with anegative bias. Plasma processing conditions can be set as, e.g., 1000 VDC voltage, 150 W RF power, 10 mT pressure, and 10 sccm flow of an inertgas such as argon. A plasma process time of less than 1 min is needed toproduce an oxide surface charge. Less than about 100 Å of oxide areetched during this duration for the process conditions given. With aplasma initiated, the metal electrode and insulated wafer together forma capacitor in which positive electrostatic charge is accumulated on theoxide layer surface.

The CO₂ ice jet cleaning procedure described previously can also beemployed to impart a positive electrical charge to an oxide surface. Itis required that the wafer be heated during the ice jet cleaning toproduce the positive charge state. This can be accomplished, asdescribed above, by maintaining the wafer on a thermally conductivesupport block to which a heater is connected during sweep of an ice jetover the oxide layer surface. With this configuration, the ice jetprocess given above results in positive oxide surface charging.

A positively charged oxide surface can be produced by a variety of othertechniques. In one particularly simple technique, a sheet of clean roomgrade Kimwipes® or other such anti-static material can be wiped acrossthe surface of the oxide layer to impart a positive electrical charge tothe layer. The anti-static coating of the material is found to generatea positive charge on the oxide surface.

These examples provide a description of several efficient and simplecharging techniques. It is to be understood that other techniques can beemployed to impart a positive electrical charge to an oxide surface, andthe invention is not limited to a particular technique.

Electrostatic charge release of an oxide layer can be accomplished inaccordance with the invention by a variety of techniques. In a firsttechnique, charge is released by a water rinse of the oxide surface. Theelectrical conductivity of water is understood to enable the release ofcharge from the layer. In a further charge release technique, the CO₂ice jet cleaning process described earlier is carried out on an unheatedwafer, i.e., condensation is permitted during the ice jet process. As aresult, charge is removed by the adsorbed species and swept away withthe species by the jet stream. It is to be recognized that othertechniques can be employed to release charge from an oxide surface. Theinvention is not limited to a particular charge release technique.

EXAMPLES 20-21

A thermal oxide layer was formed on a plurality of silicon wafers. Halfof the oxide layers were treated by wiping a Kimwipes® cloth over theirsurfaces to impart a positive electrical charge to the surface. Theremaining oxide layers were subjected to a CO₂ ice jet cleaning whileunheated, to release electrostatic charge from their surfaces. Onequarter of the charge oxide layers and one quarter of the unchargedoxide layers were subjected to HF vapor process conditions of a partialpressure of HF vapor of 10 T, a partial pressure of water vapor of 4 T,a total flow rate of about 500 sccm and a total pressure of about 125Torr at a variety of temperature conditions. One quarter of the chargeoxide layers and the final quarter of uncharged oxide layers weresubjected to HF vapor process conditions of a partial pressure of HFvapor of 20 T, a partial pressure of water vapor of 8 T, a total flowrate about 500 sccm and a total pressure of about 125 Torr.

The etch rate of each of the vapor processes was determined based onellipsometric measurements taken during the processes. FIG. 13 is anArrhenius plot of etch rate, in Å/min, as a function of processtemperature, for the two different process conditions and the twodifferent surface charge conditions. It is clearly indicated from theplot that for the sub-monolayer and monolayer process regimes, thepositive surface charge resulted in an etch rate increase; this increasewas most dramatic for the sub-monolayer regime temperatures, at which arate increase of a factor of almost 100 is found. The positive surfacecharge had very little, if any, impact on the etch rate in themultilayer regime, however. These results were reflected in the plots ofFIGS. 5, 6, and 10 described above, where it was indicated that aninitial positive polarity electrical surface charge was produced priorto the etch processes.

It is believed that the positive oxide surface charging enables directionization of vapor HF adsorbed on the oxide surface, leading to anincrease in the rate of the oxide etch reaction. This is consistent withthe fact that the monolayer regime is dependent on surface reactionkinetics and that the sub-monolayer regime is very strongly dependent onsurface reaction kinetics.

EXAMPLES 22-23

The plot of FIG. 13 also highlights the impact of positive surfacecharging on the vapor process dependence on HF vapor partial pressure.For the process temperatures in the sub-monolayer and monolayer regimesbetween approximately 125° C. and 150° C., it is found that for theelectrically charged oxide layers, a doubling of HF and water partialpressures from 10 T to 20 T and 4 T and 8 T, respectively, results in adoubling of the etch rate. Conversely, for the uncharged oxide layers, adoubling of the HF and water partial pressures from results in an etchrate increase by a factor of between three and four.

This correspondence was further investigated. Thermal oxide layers wereproduced on silicon wafers and all were positively charged by wipingtheir surfaces with an anti-static cloth. The wafers were then subjectedto w HF vapor processes carried out in the sub-monolayer regime, at atemperature of 145° C., a total flow rate of 500 sccm, and a partialpressure of water vapor of 4 T. The partial pressure of the HF vapor wasvaried from process to process. The etch rate of each process wasdetermined based on ellipsometric measurements taken during eachprocess.

FIG. 14A is plot of oxide etch rate, in Å/min, as a function of HF vaporpartial pressure, measured for the positively charged oxide layers. Thismatrix of experiments confirms that in the charge-activated, high-etchrate state, the etch rate of the sub-monolayer and monolayer regimes isdirectly proportional to the HF partial pressure, following simple firstorder Langmuir adsorption kinetics.

The data from FIG. 14A is replotted in FIG. 14B, which also includesetch rate data for a process employing the same process conditions asthose relating to the data of FIG. 14A but with no water vaporintroduced during the vapor process. Under these conditions and for lowHF partial pressures, there is seen to be no substantial differencebetween the etch rates. This indicates that for positively charged oxidelayers being etched under these conditions in the sub-monolayer ormonolayer regimes, the effect of water vapor partial pressure isnegligible for low pressures and for low water vapor pressures. It is tobe recognized, however, that under other conditions, the water vaporpartial pressure significantly affects the etch rate in the monolayerand sub-monolayer regimes with positive charge enhancement of the etchrate. The plotted data of FIGS. 5, 6, and 10 confirm this, given that apositive charging of the oxide surface was used to obtain the highetching rate state of those reported experiments.

EXAMPLE 24

The dependence of etch rate on HF vapor partial pressure was alsodirectly investigated for the sub-monolayer and monolayer regimes ofetching of uncharged oxide layers. Thermal oxide layers were produced onsilicon wafers and subjected to a water rinse to enable release ofcharge from the oxide surface. The oxide layers were then exposed to HFvapor conditions in the sub-monolayer regime, consisting of a totalpressure of 125 T, a temperature of 145° C., a total flow rate of 500sccm, and no water vapor. Each wafer was processed at a different HFvapor pressure.

FIG. 15 is a plot of etch rate, in Å/min, as a function of HF pressure,determined based on ellipsometric measurements taken during the etchprocesses. As indicated by the plot, there is found to be a second orderdependency of etch rate on HF vapor pressure for the non-charged oxidelayers. This confirms the results plotted in FIG. 13, which alsoindicate a second order etch rate-HF vapor pressure dependency for theuncharged oxide layer conditions.

EXAMPLE 25

Characteristics of uncharged oxide etch processes in the sub-monolayerand monolayer regimes were further investigated. Thermal oxide layerswere produced on silicon wafers and were water-rinsed to release anyelectrical charge that might be present. The wafers were then exposed tosub-monolayer and monolayer HF vapor conditions of a temperature of 90°C., a total pressure of 125 T, a total flow rate of 500 sccm, and acombined reactant partial pressure of 28 T. Each wafer process employeda different reactant ratio.

FIG. 16 is a plot of etch rate, in Å/min, as a function of reactantratio, based on ellipsometric measurements taken during the etchprocesses. For the non-activated, low-etch rate process data plotted, itis found that the etch rate is not impacted by the ratio of reactantpartial pressures. This is in great contrast to the results obtained forthe charged, activated process data presented in FIGS. 14A-14B, where itis indicated that the HF vapor partial pressure strongly influences theetch rate. Such is understandable given that the positive surfacecharging is found to directly ionize adsorbed HF species; a higherdegree of HF adsorption would correspond to a higher etch rate.

These examples highlight the very clear process-dependent effects thatcan be relied on to distinguish the activated etch state from thenon-activated etch state for the sub-monolayer and monolayer etchregimes. Positive charging of an oxide layer to an activated stateresults in an etch rate increase by a factor of about 5, and a shiftingof the etch rate from a second order to a first order dependency on thepartial pressure of HF vapor. The activated state is found to be closelyimpacted by HF vapor partial pressure, with no effect by water vaporpressure, while the non-activated state is found to be not impacted bythe reactant partial pressure ratio. The positive charging is found togreatly impact the sub-monolayer and monolayer regimes but to havesubstantially no impact on the multilayer regime.

EXAMPLES 26-28

The impact of positive-polarity oxide layer charging was furtherinvestigated for HF vapor processes employing methanol and isopropylalcohol instead of water vapor. Thermal oxide layers were grown onsilicon wafers, and on half of the layers was produced apositive-polarity charge by wiping with a Kimwipese® cloth. HF vaporprocesses were carried out, with process conditions of the temperatureat 95° C., the total flow rate of 500 sccm, a total pressure of 125 T,and a partial pressure of 20 T. Processes were carried out employingeither water vapor, methanol, or isopropyl alcohol, all at a partialpressure of 8 T.

FIG. 17 is a plot of etch rate, in Å/min, for each the water vapor,methanol, and isopropyl alcohol processes, each for a non-activated,uncharged oxide layer and for an activated, positively charged oxidelayer. The results indicate that the largest increase in etch rate bycharge activation is achieved for an HF-water vapor process. In thenon-activated state, the HF-isopropyl alcohol process results in thehighest etch rate, while for the activated state, the HF-water vaporprocess results in the highest etch rate. This experiment verifies thata step of electrically charging or discharging an oxide layer prior toan HF vapor process can be employed to control the etch rate of thevapor process, including any of water vapor, methanol, or isopropylalcohol with the HF vapor.

Turning to processes for producing the charged, activated state for themultilayer regime, the inventors herein have discovered that a negativeelectrical charging of an oxide surface results in activation of thesurface for HF vapor etching in the multilayer regime. This isunderstood to enhance etch rate in the multilayer regime by enhancingthe negative potential of the oxide surface, which in turn enhancesadsorption of the polar reactant molecules on the oxide surface. Suchenhanced adsorption is found to enable the formation of a multilayerthat is thicker than that produced without oxide layer charging, and isdue to the electrostatic enhancement of the adsorption energy of thesemolecules to the surface. The resulting thicker multilayer exhibits anetch rate higher than a thinner multilayer, as expected.

A negative charge can be imparted to an oxide surface by a variety oftechniques provided by the invention. In a first technique, the plasmaexposure process described above is employed with the DC voltage biaspolarity reversed. Here a positive DC bias placed on the lower electrodeof a plasma chamber on which a wafer is supported and insulated resultsin the generation of negative charge on the oxide layer surface. Theplasma conditions given above, e.g., 10 sccm of argon flow, 150 W RFpower, 10 mT pressure, and 1000 V DC bias, can be employed here forproducing negative surface charge.

An asymmetric diode plasma reactor in which the wafer is mounted on thesmaller electrode can also be employed to impart a negative electricalcharge to an oxide layer surface. In this configuration, in, e.g., areactive ion etcher or sputtering chamber, the wafer is mounted on thesmaller electrode of the chamber to achieve an RF biasing of theelectrode and resulting charging of the wafer surface. Using electrodesof differing areas in contact with the plasma results in a negativecharge build up on the surface of an oxide layer on the wafer. Processconditions of, e.g., 30 W RF power to the bottom/smaller electrode, 30mT of pressure, and 1 min of processing time in a sputtering chamber issufficient to impart a negative charge to the surface of an oxide layer.

Negative charge can also be produced on an oxide layer by exposing thelayer to a low energy electron beam. Conditions of, e.g., anaccelerating voltage of about 100 eV and an emission current of about1000 μA can be employed. Exposure of the surface to the beam conditionsresults in a negative charging of the oxide.

Negative charge build-up on an oxide layer surface can further beproduced by exposure of the oxide layer surface to an HF vapor processfollowed by storage in a vacuum environment for at least about 2 hours.The preliminary HF vapor process can be employed as an initial partialetch of the layer. The HF vapor process is found to fluorinate the oxidesurface, with the resultingly exposed fluorine atoms becoming negativelycharged by electrons in the vacuum environment, e.g., from an ion gaugeor low energy electron beam.

W It is to be recognized that there are a wide range of other processesfor producing a negative surface charge on an oxide surface. Theinvention is not limited to a particular technique for producing anegative-charge build-up on an oxide layer surface.

EXAMPLES 29-30

Thermal oxide layers were produced on silicon wafers. Half of the oxidelayers were rendered in a negative-charge activated state by exposingthe oxide layers to a plasma environment of an argon flow rate of 20sccm, a pressure of 30 mT, an RF power of 30 W, and a positively biasedDC voltage applied to an electrostatically-clamped wafer holder withabout 600 volts. The other oxide layers were rendered charge-free by awater rinse.

The oxide layers were subjected to HF vapor processes, all with an HFvapor partial pressure of 10 T, a water vapor partial pressure of 4 T, atotal pressure of about 125 T and a total flow rate of 500 sccm. Eachprocess was carried out at a different temperature.

FIG. 18 is an Arrhenius plot of etch rate, in Å/min, as a function oftemperature, as-determined based on ellipsometric measurements takenduring the vapor processes. As indicated by the data of the plot, etchprocesses in the sub-monolayer and monolayer regimes were not impactedby the negative oxide layer surface charging. The multilayer regime isfound, however, to be significantly enhanced by the negative oxide layersurface charge.

It is also found, as further indicated by the plotted data, that thenegative oxide layer surface charge results in an increase of thetemperature of transition from the monolayer regime to the multilayerregime by about 20° C. This transition temperature shift is understoodto result from an enhancement of reactant adsorption by reduced reactantvapor pressures for the etch processes carried out on the charged oxidesurface.

It is contemplated in accordance with the invention that the variousoxide layer charge conditions described above be directly measured. Suchcan be accomplished by conventional means for enabling a confirmation ofthe polarity of charge produced on a given oxide layer.

The examples given above demonstrate the advantages provided by theinvention for controlling particular operating regimes of HF vaporprocesses. Sub-monolayer, monolayer, multilayer, and condensed layeroperating regimes have been discovered by the inventors, and distinctivecharacteristics of each regime have been described to enableidentification of the regimes. Specific pre-process steps are providedfor enabling further control of the etch rate and for compensating forprior contamination.

For many applications, it is preferred in accordance with the inventionto control an HF vapor process to proceed in the sub-monolayer ormonolayer regimes. Being surface reaction rate limited, these regimesenable highly uniform etching, resulting in elimination of surfacepitting and corresponding “haze” that is conventionally associated withetching in the multilayer regime. Furthermore, the formation of etchresidue particulates is eliminated in the sub-monolayer and monolayerregimes by the complete and immediate evaporation of process productsduring an etch reaction.

Processes in the sub-monolayer and monolayer regimes are foundunexpectedly to proceed uniformly, even on silicon wafer surfacesexhibiting prior hydrocarbon contamination, when a first step of dry icecleaning is performed on the wafer surface in accordance with theinvention. Although the ice jet cleaning may not completely removesurface contaminants, it is found that the sub-monolayer and monolayerprocessing regimes of the invention are rendered repeatable andpredictable by a first ice jet cleaning step, contrary to conventionalwisdom.

The sub-monolayer and monolayer HF vapor process regimes arecharacterized by an oxide etch rate on the order of about 5 nm/min,enabling a high degree of control of processing, e.g., to avoidundercutting of thick films when removing thin native oxide layers. Inaddition, positive charging of an oxide surface can be carried out inaccordance with the invention to increase the sub-monolayer andmonolayer etch rates while maintaining the benefits of these etchregimes. The selectivity of undensified TEOS oxide and thermal oxide tosub-monolayer and monolayer etching conditions is about 2-3:1, enablingthe cleaning of sandwich structures without excessive undercutting ofthe TEOS film. The sub-monolayer and monolayer etch regimes are alsofound to not attack metal, e.g., aluminum or copper.

These advantages enable a process sequence, provided in accordance withthe invention, in which a vacuum cluster tool is employed for viaetching, contact cleaning of native oxide, and metal deposition.Referring to FIG. 19, there is shown such a configuration 100. Siliconwafers 102 are transferred between various vacuum chambers under vacuumvia a robot 103 through a transfer chamber 105. The vacuum processchambers can include, e.g., a plasma etch chamber 107, ashing chamber109, dry cleaning stations 111, 113, and a metal deposition chamber 115.

A wafer 102 having an oxide layer 104 formed upon it, can be processedto include a patterned photoresist layer 106 for defining a metal via.The wafer is then introduced to the cluster system and first processedin the plasma etch chamber 107 to etch the via pattern in the oxidelayer. The wafer is then transferred to the ashing chamber 109 forremoving the photoresist 106. At this point in the process, photoresistresidue and metallic impurities 108 likely exist on the oxide layer 104and a native oxide layer 110 likely exists on the surface of the siliconwafer. Such can be removed in situ by transferring the wafer to the drycleaning chambers 111, 113 for a pretreatment of, e.g., UV exposure orice jet cleaning, if desired and then removal of the residue and nativeoxide by an HF vapor process in the sub-monolayer or monolayer regime.Thereafter, the wafer is immediately transferred to the metal depositionchamber 115 for deposition of a metal layer 112 on a freshly cleanedwafer and oxide surface and via.

Referring to FIG. 20, in a further example of a vacuum cluster toolsequence provided in accordance with the invention, there is providedthe ability to carry out metal line etching, polymeric residue removal,and interlayer dielectric deposition in situ under vacuum in one metaletch system 119. As with the system of FIG. 19, there is here enabledwafer transfer between various vacuum chambers under vacuum via a robot103 through a transfer chamber 105. The vacuum process chambers caninclude, e.g., a plasma etch chamber 107, ashing chamber 109, drycleaning stations 111, 113, and an interlayer dielectric depositionchamber 117.

A wafer 102 having, e.g., an oxide layer 120 has formed upon it a metallayer 122, e.g., a layer of aluminum. A layer of photoresist 124 isdeposited and patterned corresponding to the metal line pattern desiredfor the metal layer 122. The wafer is introduced to the cluster system119 and first processed in the plasma etch chamber 107 to pattern themetal layer 122. Typically, at the completion of the metal plasma etch,polymeric residue 124 is built up on the sidewalls of the etched metallines. The wafer is then introduced through the cluster system to theashing chamber 109 for removal of the photoresist. Even after thisashing step, polymeric sidewall residue is likely to remain.

At this point the wafer is transferred through the cluster system to thedry cleaning chamber 111 for a pretreatment of, e.g., UV exposure or icejet cleaning, if desired, or Cl treatment. A hard polymer sidewallresidue, typically consisting primarily of oxide, is often found toexist at this point. Removal of this residue is then carried out bytransfer of the wafer to the second: dry cleaning chamber 113 forcompletion of an HF vapor process in the sub-monolayer or monolayerregime. Thereafter, the wafer is immediately transferred to theinterlayer dielectric deposition chamber 117 for deposition of aninterlayer dielectric over the patterned metal layer 122.

As can be recognized, the functionality of the two vacuum clustersystems just described can be integrated to provide a large scale vacuumsystem for metal deposition, patterning, and interlayer dielectricdeposition. Both of the two vacuum systems, and an integrated system inparticular, are cost effective, environmentally friendly, and highlyefficient. The relatively high temperature operating conditions of thesub-monolayer and monolayer HF vapor process regimes of the inventionenable completely dry cleaning processes for the cluster systems. Theseregimes are well-suited for a vacuum cluster configuration because theyinhibit condensation conditions, thereby enabling rapid vacuum pumpingand eliminating the potential for system component corrosion.

For many applications, including the vacuum cluster configurations justdescribed, it can be preferred to operate an HF vapor process undersub-monolayer, rather than monolayer, conditions. The sub-monolayerprocess regime was demonstrated in the experimental examples given aboveto be less temperature-sensitive than the monolayer regime. Thisdecreased temperature sensitivity results in a broadening of theallowable process conditions for the regime, leading to greater ease ofprocess control. Lack of process control in the monolayer regime canresult in process nonuniformity and the possible formation of localizedmultilayer regime regions, these having dramatically higher etch ratesand the likely formation of surface pitting and roughness as well aspost process residue. Small-sized and large-aspect ratio features areparticular susceptible to such process artifacts. The controllable,slow-etch rate conditions of the sub-monolayer processing regime arefound to inhibit these effects and therefore to be a more robust,reliable operation for manufacturing conditions. It is to be recognized,however, that the invention does not entirely dismiss monolayer andmultilayer HF vapor processes. Particularly when combined with aninitial surface charging or discharging step or an ice jet cleaningstep, these processes can be successfully employed for a wide range ofapplications.

It is further to be recognized based on the discussion above that the HFvapor processes of the invention can be applied to a wide range ofmaterials beyond the silicon, silicon dioxide and native silicon dioxidematerials described. Other oxides and other semiconductor materials canbe processed in accordance with the invention. Polymeric residue can beremoved from metal and other lines, and carbon and other contamination,including residues, can be removed from substrate surfaces. The processconditions are not limited to semiconductor wafers; substrates ofvarying size and geometry can be accommodated by the processes of theinvention.

This discussion highlights the wide range of applications of the HFvapor processes of the invention. Truly “all dry” etch and cleaningprocesses are provided, whereby semiconductor processing efficiency,repeatability, ease of control, and environmental friendliness are allenhanced.

It is recognized, of course, that those skilled in the art may makevarious modifications and additions to the HF vapor processes of theinvention without departing from the spirit and scope of the presentcontribution to the art. Accordingly, it is to be understood that theprotection sought to be afforded hereby should be deemed to extend tothe subject matter of the claims and all equivalents thereof fairlywithin the scope of the invention.

We claim:
 1. A method for etching oxide on a semiconductor substrate,comprising the steps of: producing a positive electrical charge on theoxide; and subsequent to the positive electrical charge production,exposing the previously positively charged oxide on the substrate tohydrofluoric acid vapor and water vapor in a process chamber held attemperature and pressure conditions that are controlled to form on thesubstrate no more than a saturated monolayer of etch reactants andproducts produced by the vapor as the oxide is etched by the vapor.
 2. Amethod for etching oxide on a semiconductor substrate, comprising thesteps of: producing a positive electrical charge on the oxide; andsubsequent to the positive electrical charge production, exposing thepreviously positively charged oxide on the substrate to hydrofluoricacid vapor and methanol vapor in a process chamber held at temperatureand pressure conditions that are controlled to form on the substrate nomore than a saturated monolayer of etch reactants and products producedby the vapor as the oxide is etched by the vapor.
 3. A method foretching oxide on a semiconductor substrate, comprising the steps of:producing a positive electrical charge on the oxide; and subsequent tothe positive electrical charge production, exposing the previouslypositively charged oxide on the substrate to hydrofluoric acid vapor andisopropyl alcohol vapor in a process chamber held at temperature andpressure conditions that are controlled to form on the substrate no morethan a saturated monolayer of etch reactants and products produced bythe vapor as the oxide is etched by the vapor.
 4. The method of any ofclaims 1, 2, or 3 wherein the process chamber temperature and pressureconditions are controlled to form on the substrate no more than asub-monolayer of etch reactants and products produced by the vapor asthe oxide is etched by the vapor.
 5. The method of any of clam 1, 2, or3 wherein the positive electrical charge is produced on the oxide byexposure of the oxide to an electron beam.
 6. The method of any ofclaims 1, 2, or 3 wherein the positive electrical charge is produced onthe oxide by exposure of the oxide to ultraviolet light through anelectrically-biased metallic screen.
 7. The method of any of claims 1,2, or 3 wherein the positive electrical charge is produced on the oxideby exposure of the oxide to a plasma environment wherein the substrateis capacitively biased by a negative-polarity DC voltage.
 8. A methodfor etching oxide on a semiconductor substrate, comprising the steps of:producing a negative electrical charge on the oxide; and subsequent tothe negative electrical charge production, exposing the previouslynegatively electrically charged oxide on the substrate to hydrofluoricacid vapor and water vapor in a process chamber held at temperature andpressure conditions that are controlled to form on the substrate no morethan a multilayer of etch reactants and products produced by the vaporas the oxide is etched by the vapor.
 9. The method of claim 8 whereinthe negative electrical charge is produced on the oxide by exposure ofthe oxide to a plasma environment wherein the wherein the substrate iscapacitively biased by a RF voltage.
 10. The method of claim 8 whereinthe negative electrical charge is produced on the oxide by exposure ofthe oxide to a plasma environment wherein the substrate is capacitivelybiased by a positive-polarity DC voltage.